1. Field of Invention
The invention relates to a rotation speed control device and method, and, in particular, to a rotation speed control device and method for controlling the rotation speed of a fan.
2. Related Art
Most electronic systems are equipped with a fan for heat dissipation so as to prevent some elements in the electronic systems from being over-heated and producing glitches.
As shown in FIG. 1, a conventional fan 1 includes a motor 11. If the motor 11 is a DC brushless motor, the rotation speed and voltage VM of the motor 11 are directly proportional. To control the rotation speed of the fan 1, the motor 11, which is electrically connected with the power source VCC, may further couple to a linear voltage regulator 12 so as to change the voltage of the motor 11. By changing the voltage VT of the linear voltage regulator 12, the voltage VM of the motor 11 as well as the rotation speed of the fan 1 can be controlled. In more detailed, the voltage VM of the motor 11 equals to the power source VCC minus the voltage VT of the linear voltage regulator 12 (VM=VCC−VT). For example, when the voltage VT of the linear voltage regulator 12 increases, the voltage VM of the motor 11 decreases resulting in the slowed rotation speed of the fan 1. However, although the above-mentioned control method can easily change the rotation speed of the fan 1 by changing the voltage VT of the linear voltage regulator 12, it may cause the increased power consumption. In addition, since the linear voltage regulator 12 is constantly working and changing voltage, it generates heat easily and the heat is not easily dissipated.
To improve the above-mentioned disadvantage, as shown in FIG. 2, another conventional fan 2 includes a motor 21 electrically connected with the power source VCC and a power switch element 22. A PWM (pulse width modulation) signal is inputted to control the active/inactive period of the power switch element 22. For example, when the duty cycle of the PWM signal has a longer high-level period, the power switch element 22 has longer active period so that the rotation speed of the fan 2 becomes faster. That is, the electronic system utilizes the periods of the high level and low level of the PWM signal to control the period of ON or OFF of the power switch element 22. However, the noise caused by the switching of the power switch element 22 is obvious when the fan 2 is at a low rotation speed. As shown in FIG. 3, the switching of the fan 2 may lead to the rapid changes of the voltage (ΔV) and the current variations per unit time (di/dt), which results in higher electromagnetic interference.
Therefore, it is desired to provide a fan speed control device and method, which can decrease the power consumption, reduce the noise of switching, and avoid electromagnetic interference.